GT_PFR_FFI extracts the information on whether the image is a partial frame image (PFI) or a full frame image (FFI). Even values are PFI, odd values are FFI in general.
The /ffi keyword option can be used, and it will return a 1 if the image is an FFI and return a 0 if it is a PFI (PFI strip or observing region). The /true_ffi keyword option will return a 1 if the image is an FFI and return a 0 if it is a PFI (PFI strip or observing region). The /true_ffi option will also check if it is an FFI strip or if it is a PFI which was extracted from an FFI.